The miniaturization of recent electronic circuitry has rapidly become widespread and this trend is apparent in electronic apparatuses, particularly in portable devices.
Miniaturization has an effect on various aspects of electronic device fabrication such as, for example, the size of rechargeable batteries used in the portable devices and charging control circuitries for the batteries for housing in battery packs.
The charging control circuitry is preferably devised, therefore, to be able to mount chip-type electronic components including a semiconductor device on a printed-circuit board, which can be as small as several millimeters. The printed-circuit board is then provided with prescribed resin sealing to achieve necessary mechanical strength and reliability.
FIGS. 4A and 4B are drawings diagrammatically illustrating a known printed-circuit board, in which FIG. 4A is a plan view of the circuit board, while FIG. 4B is a cross-sectional view taken along line VV of the structure of FIG. 4A.
Referring to FIGS. 4A and 4B, the printed-circuit board is capable of mounting at least one electronic component including a semiconductor device, having external connecting electrodes on a bottom face.
On a circuit board substrate 32, a circuit pattern 36 of copper film is formed. Soldering lands 34 are also formed as electrode portions for making electrical connections to the external connecting electrodes of the mounted electronic component. A solder resist film (or insulating materials layer) 38 is formed over the entire surface of the structure so as to have openings in the regions corresponding to the specific locations of the soldering lands 34.
The solder resist film 38 is formed herein for preventing solder, used for electrically connecting the electronic components, from unduly spreading out onto neighboring soldering lands or circuit patterns 36, possibly causing short circuiting. The solder resist film 38 also serves to protect, and prevent the deterioration of, the circuit pattern 36 previously formed.
In the abovementioned structure, with the printed-circuit board mounted with electronic components and sealed with sealant resin, adhesive strength between solder resist film 38 and the sealing resin is smaller than the strength between the substrate 32 and the resin, and the mechanical strength of solder resist film 38 itself is not satisfactory. This may give rise to the separation or peeling of the sealing resin from solder resist film 38, or to the cracking of the film 38 within.
Peeling of the sealant resin from the substrate of circuit board can give rise to several adverse effects, such as moisture permeating into the interior and deterioration of the reliability of the device.
In order to alleviate such difficulties, a method, in which the solder resist film 38 is provided with a number of circular or hexagonal-shaped openings 40 at least 0.5 millimeter apart from each other, is illustrated in FIGS. 5A and 5B. The sealant resin is brought into direct contact with the substrate to a greater degree and adhesive strength increases between the sealant resin and the substrate (for example, Japanese Laid-Open Patent Application No. 9-205164).
In the method of the abovementioned '164 application, however, it is hard for such a small circuit board substrate to accommodate many openings 40 so as to mount a plurality of chip-type electronic components.
Further, it is difficult to provide the openings 40 of the number large enough to achieve satisfactory adhesive strength between the sealant resin and the substrate.
Another method has also been disclosed, in which a sealant resin pattern is formed directly on a circuit board substrate without interposing a solder resist pattern (Japanese Laid-Open Patent Application No. 8-111477).
In this method, either no solder resist pattern is formed at all, or a pattern is formed and subsequently removed prior to resin sealing.
In the case of the '477 application, several drawbacks arise, including frequent short-circuiting caused by the formation of solder bridges during the steps of printing cream solder for soldering electronic components. Additionally, an increase in production costs results from appending a process step for removing solder resist following the soldering.
It is therefore desirable to overcome the abovementioned difficulties and achieve satisfactory adhesive strength between the sealant resin and the substrate even with interposing a solder resist layer therebetween.